1. Field of the Invention
This invention relates to a process for producing lignin reinforced polymer compositions. More particularly, this invention relates to a process for forming lignin-latex coprecipitates and gas drying the coprecipitated lignin/latex slurry to leave a lignin reinforced polymer free of inorganic salts.
2. The Prior Art
Lignin has been found to be an effective reinforcing agent for rubber when incorporated therein by a coprecipitation process as shown in U.S. Pat. No. 2,608,537 to Pollack. Several disadvantageous characteristics of this and other processes have, however, limited any large scale commercialization. Disadvantages inherent in the coprecipitation processes include those connected with the physical characteristics of the lignin rubber coprecipitate, removal of inorganic salts and low melting point of the lignins used.
Lignin-rubber coprecipitates, as ordinarily produced according to prior art techniques, generally have the nature of a paste or mud which is difficult to process to obtain a dry cake or crumb, thus requiring high temperatures or long drying times and the need to be filtered. The particles of the coprecipitate, due to their very small size, tend to fill the pores of any filter medium, resulting in very low filtration and washing rates. When filtered and washed, the particles do not possess much cohesiveness and yield a crumbly filter cake which is difficult to handle during subsequent drying operations. The particles in the filter cake are non-compressible and retain large quantities of water which cannot be expressed by mechanical means and must be removed by application of heat. In general, the solids content of coprecipitates will range from 25 to 35%.
U.S. Pat. No. 3,167,523 to Dimitri discloses a method to improve the processing of modified lignin-rubber coprecipitates by heating a slurry prior to filtration. This added step is absolutely necessary if reasonable filtration rates are to be obtained. The modified lignins are used because they have melting points that are above the temperatures encountered in rubber processing, but these lignin-rubbers are high in inorganic salt content.
U.S. Pat. No. 3,312,643 to Ball discloses still another process for treating lignin-rubber coprecipitates which includes mixing formaldehyde with a slurry of lignin-rubber particles, and heating to cause the lignin and formaldehyde to react.
U.S. Pat. No. 3,697,497 to Falkehag discloses hexamethylene tetramine modifications of lignin used to make a high melting, high surface area lignin by a relatively simple, straight forward reaction which includes spray drying. However, that process requires heat treatment prior to filtration when it is to be used in lignin-rubber coprecipitates.
U.S. Pat. No. 3,808,192 to Dimitri discloses a process for the production of high surface area lignins by spray drying which requires a partial coagulation of lignin prior to spray drying.
While some of the processes discussed herinabove have been proposed for improvements of the physical characteristics of lignin-rubber coprecipitates, these processes have either not completely solved the problems or have resulted in degradation of the properties of cured rubber prepared from coprecipitates. Consequently, it is the primary object of this invention to provide a process for making lignin reinforced rubber and resin polymers whereby heat treatment and filtration steps are eliminated, thus greatly simplifying operations. Another object of this invention is to provide a process whereby lignin-latex coprecipitates can be obtained without degrading the properties of the cured polymer. An additional object of this invention is to provide a process for making reinforced rubber and resin polymers using a modified lignin having a high melting point. Still another object of this invention is to provide a process whereby a hexamethylene tetramine modified lignin is coprecipitated with a latex using carbon dioxide or sulfur dioxide and gas dried.
Other objects, features and advantages of this invention will become evident from the foregoing detailed description of the invention.